(1) Field of the Invention
The present invention relates generally to anodizing systems including a coating thickness monitor and, more particularly, to a system for regulating an anodized coating thickness on a substrate as it is being formed as well as measuring the coating thickness subsequent to its formation.
(2) Description of the Prior Art
The coating of metallic substrates such as aluminum and zinc using anodizing is known. Anodizing is done for practical and aesthetic reasons. From a practical perspective, the creation of a coating on the surface of a metallic substrate contributes to an anodized product's wear resistance, corrosion resistance, and oxidation resistance. From an aesthetic perspective, the creation of a coating including a dye for coloration on the surface of a metallic substrate contributes to an anodized product's consumer appeal. In both industrial and aesthetic applications, it is desirable to control the thickness of the anodized coating as well as the consistency over a given surface area.
Commonly, coating thickness is determined by destructive methods. For example, in a batch anodizing system, control coupons made of the same material as a product to be anodized are included in the anodizing bath. At intermediate times during the anodizing process a control coupon is removed from the bath and destroyed in a manner that permits determining the coating thickness.
One destructive method includes mounting a control coupon in a Bakelite cross-section, polishing the mounted coupon to a mirror finish and examining the polished cross-section using an optical microscope to determine the coating thickness. A second destructive method includes cutting or breaking a control coupon to expose a cross-section and examining the cross-section using scanning electron microscopy to determine the coating thickness. These destructive methods are cumbersome in production.
Both destructive methods delay production because of the time taken to remove and prepare control coupons for determining coating thickness. During the delay, the bath is idle. An alternative is to remove the product from the anodizing bath while determining coating thickness and replace it with a second product and corresponding control coupons. In this case, storage area for the product removed from the bath during a coating thickness determination would be required at the production site.
Although using an anodizing bath alternatively with multiple products provides a solution to production delay, coating flaws can be introduced by bath chemistry changes and surface contagion during storage. That is, the different bath chemistry when the product is reintroduced after the coating thickness determination for further anodizing may create a distinct mismatched interface with the original coating.
During storage, the original coating on the product may also be damaged during removal from and replacement into the anodizing bath. Particulate matter such as dust also may attach to the surface to introduce further interfacial flaws between the original coating and the further coating.
The above destructive methods have another serious flaw, namely, that the determined coating thickness is that of a control coupon and not of the product. Thus, the coating thickness of the product is only an estimate and the coating thickness consistency over the entire surface of the product is unknown.
Thus, there remains a need for a new and improved anodizing system that includes a coating thickness monitor that nondestructively determines the coating thickness on a product, while at the same time, has the ability to control the anodizing system. There also remains a need for a coating thickness monitor that nondestructively determines the coating thickness on an anodized product.